A、One-half. B、One-fourth. C、One-third. D、Two-thirds. C题目问的是双胞胎完全一样的概率是多少。由定位句可知，双胞胎完全一样概率是三分之一。故选C。

Although twins have always been a source of curiosity, they are not so uncommon statistically, since they occur once in every 86 births. About one-third of all twins are identical, or single-egg twins. Identical twins have the same genes and, hence, the same sex, hair, eyes, blood type, and bone and tooth structure. Some identical twins are mirror images of each other. For example, one may be left-handed, the other right-handed. As young children, some identical twins may develop their own private language. Identical twins have a special intuition, and they often seem to think and dress alike even though they are away from each other. In fact, even when they are separated at birth and raised apart, identical twins develop startling similarities. They may pursue the same careers, have the same interests, or die within days of each other.
    Q22: How many twins occur every 86 births?
    Q23: About what proportion of twins are identical?
    Q24: When they are young, what do some identical twins develop?